The TARDIS is the Doctor and other Time Lord's Timetravelling spaceship. Disguised as a 1960s Police Box the TARDIS could go unseen almost anywhere. The TARDIS is an acronym which Susan explained in an unearthly child.

'I made up the name TARDIS from the initials of TimeAnd RelativeDimension In Space' -Susan

A properly piloted TARDIS can transport anyone who is in it to any place in time or space. The interior of a TARDIS is much bigger than its exterior 'Its bigger on the inside' . The Doctor pilots a Type 40 Mark 1 TARDIS. The TARDIS' chameleon circuit is broken, leaving it stuck in the style of a London Police Box after a visit to London in 1963.

The TARDIS was already old when The Doctor 'borrowed' it from a repair shop in Gallifrey, but it's age is not specified. According to the novel: The Gallifrey Chronicles by Lance Parkin, The Doctor's TARDIS was previously owned by a Time Lord called Marnal, who was like The Doctor. In 'The Doctor's Wife' he claimed he has been travelling for 700 years in the TARDIS, in the 'Journey to the Centre of the TARDIS' he claimed he has been travelling for 900 years and also in the 'Empty Child' he said he has had '900 years of phone box travel'.